Field of the Invention
The present invention relates generally to an improved finned tube heat exchanger, and more specifically to a finned tube support member and a radial bumper support means for reducing heat exchanger damage, efficiency loss, and failure due to vibration and shock loading.
In recent years there has been substantial interest in developing superconductive propulsion systems for marine vehicles. Modern marine vehicles have critical component restrictions. Size, weight and complexity are important considerations. A superconducting propulsion system is highly desirable because it is compact and light weight as compared to a comparable, traditional motor-generator system.
Superconducting propulsion systems incorporate helium liquefiers which operate at cryogenic temperatures and require a high efficiency heat exchanger. The magnet in a superconducting system must be cooled to an operating temperature approaching 0 degrees Kelvin. The system as a whole operates in normal room temperature. The helium liquefier therefore requires a heat exchanger which is highly efficient throughout a range from about 300 degrees Kelvin to about 4 degrees Kelvin.
A heat exchanger design presently used in other cryogenic systems, the conventional finned tube counter-flow heat exchanger, is unsatisfactory for shipboard operations. When a conventional finned tube counter-flow heat exchanger is subjected to the vibration and shock loading of the shipboard environment, the adjacent windings of the finned tube compress longitudinally and make contact. This creates a thermal short between the adjacent windings and seriously degrades the thermodynamic performance of the heat exchanger.
Prior attempts to prevent this thermal shorting have been unsuccessful. It is known in the conventional heat exchanger art to use a compliant spacer between the adjacent windings. This compliant spacer is satisfactory in other cryogenic applications, but not for shipboard operations. When subjected to the vibration and shock loading of shipboard operation, the adjacent windings of the finned tube wear through the compliant spacer, make contact, and cause thermal shorting. Attempts to prevent this thermal shorting by affixing the finned tube to the wall of the heat exchanger shell have also failed. Typically, the finned tube was soft soldered to the inner wall of the heat exchanger with random or periodic welds. These welds cause thermal shorting and obstruct the counter-flow. Furthermore, these welds are difficult to prepare because of the finned tube structure. The fins of the finned tube are fragile, and the welds are therefore prone to fail when subjected to the vibration and shock loading associated with shipboard operations.
Previous attempts to prevent the longitudinal compression of the finned tube windings are also unsatisfactory. Brenner, in U.S. Pat. No. 3,921,708, discloses the use of an expansion limiter in the form of a supporting jacket. The jacket, a helical channel with a rectangular cross section, is formed by a helical fin affixed to the inner and outer walls of the cylindrical heat exchanger shell. Each winding is firmly held within the supporting jacket by a four point support system. The four point support is provided by the inner wall, ridges located on each side of the support jacket fin, and set screws, located in the outer wall of the heat exchanger shell. The support jacket restricts the counter-flow of the heat exchanger. It causes the flow to process helically through the jacket, perpendicular to the finned tube fins. For peak efficiency, the flow should travel parallel to the finned tube fins. Brenner's device also requires set screw adjustments. It therefore reduces the efficiency and increases the complexity of the finned tube heat exchanger.
Another source of efficiency loss caused by the vibration and shock loading of a cryogenic finned tube counter-flow heat exchanger, is the damage and thermal shorting due to contact between the concentric cylindrical shells of the heat exchanger and insulating shell. The heat exchanger of a helium liquefier often comprises a series of concentric heat exchanger shells or shell segments. These heat exchanger shells are further contained in a concentric insulating shell, maintained at a vacuum approaching 1 micron of mercury to provide maximum thermal insulation. One end of the heat exchanger shell is rigidly held concentric to the insulating shell, but the other end of the heat exchanger shell, and any interior concentric heat exchanger shell segments, remain unsupported. When subjected to vibration and shock loading, the unsupported ends oscillate and may make contact. When the unsupported ends make contact, they cause thermal shorting. Shock loading and extended vibration cause fatigue at the rigid support and other joints of the shell walls. Sufficient material fatigue results in stress fractures and leakage between the heat exchanger shells and the vacuum-insulating shell, thereby reducing the heat exchanger efficiency.
For these and other reasons, the presently known heat exchangers are unsatisfactory for use in shipboard superconductive propulsion systems. A need exists for high efficiency cryogenic heat exchanger which retains its efficiency in the vibration and shock loading environment associated with shipboard superconductive propulsion systems.